Innate immunity and cytokines in liver injury, inflammation, and repair The liver is an organ with strong innate immunity, which plays an important role in host defense against microbial infection and tumor transformation. Emerging evidence suggests that innate immunity as well as a variety of cytokines produced by innate immune cells also contribute to the pathogenesis of acute and chronic liver diseases. Our laboratory has been actively studying the role of innate immunity and its associated cytokines in liver injury and repair. During the fiscal year, we have demonstrated that (1) STAT3 plays a hepatoprotective and an oncogenic role in liver tumorigenesis in mice;(2) In vivo liver-specific interleukin-22 expression accelerates liver regeneration and cancer development in mice;(3) Innate immunity (natural killer cells and IFN-) is suppressed in the late stages of liver injury;(4) liver regeneration is enhanced in IL-10 deficient mice after partial hepatectomy via activating hepatocyte STAT3. Hepatoprotective versus oncogenic functions of STAT3 in liver tumorigenesis. Aberrantly hyperactivated STAT3 has been found in human liver cancers as an oncogene;however, STAT3 has also been shown to exert hepatoprotective effects during liver injury. The balancing act that STAT3 plays between hepatoprotection and liver tumorigenesis remains poorly defined. In this study, the diethylnitrosamine (DEN)-induced liver tumor model and the chronic carbon tetrachloride (CCl4)-induced liver fibrosis model were both used to investigate the role of STAT3 in liver tumorigenesis. Hepatocyte-specific STAT3 knockout mice were resistant to liver tumorigenesis induced by a single DEN injection, whose tumorigenesis was associated with minimal chronic liver inflammation, injury, and fibrosis. In contrast, long-term CCl4 treatment resulted in severe hepatic oxidative damage, inflammation, and fibrosis but rarely induced liver tumor formation in wild-type mice. Despite the oncogenic function of STAT3 in DEN-induced liver tumor, hepatocyte-specific STAT3 knockout mice were more susceptible to liver tumorigenesis after 16 weeks of CCl4 injection, which was associated with higher levels of liver injury, inflammation, fibrosis, and oxidative DNA damage compared with wild-type mice. These findings suggest that the hepatoprotective feature of STAT3 prevents hepatic damage and fibrosis under the condition of persistent inflammatory stress, consequently suppressing injury-driven liver tumor initiation. Once liver tumor cells have developed, STAT3 likely acts as an oncogenic factor to promote tumor growth. In vivo consequences of liver-specific interleukin-22 expression in mice: Implications for human liver disease progression. IL-22, which acts as either a proinflammatory or anti-inflammatory cytokine in various disease models, is markedly up-regulated in chronic liver diseases, including hepatitis B and C. In this report, we demonstrate a strong correlation between IL-22 expression in the liver with active, inflammatory human liver disease. To clarify the role of IL-22 up-regulation in the pathogenesis of liver diseases, liver-specific IL-22 transgenic (IL-22TG) mice, under the control of albumin promoter, were developed. Despite elevated IL-22 serum levels ranging from 4,000 to 7,000 pg/mL, IL-22TG mice developed normally without obvious adverse phenotypes or evidence of chronic inflammation (except for slightly thicker epidermis and minor inflammation of the skin) compared with wild-type mice. Interestingly, IL-22TG mice were completely resistant to concanavalin A-induced T cell hepatitis with minimal effect on liver inflammation and had accelerated liver regeneration after partial hepatectomy. Although they did not spontaneously develop liver tumors, IL-22TG mice were more susceptible to diethylnitrosamine-induced liver cancer. Microarray analyses revealed that a variety of antioxidant, mitogenic, acute phase genes were up-regulated in the livers of IL-22TG mice compared with those from wild-type mice. CONCLUSION: These findings indicate that localized production of IL-22 in the liver promotes hepatocyte survival and proliferation but primes the liver to be more susceptible to tumor development without significantly affecting liver inflammation. Suppression of innate immunity (natural killer cell/interferon-) in the advanced stages of liver fibrosis in mice. Activation of natural killer NK cell/interferon- IFN- has been shown to play an important role in antiviral and antitumor defenses as well as antifibrogenesis. However, little is known about the regulation of innate immunity during chronic liver injury. Here, we compared the functions of NK cells in early and advanced liver fibrosis induced by a 2-week or a 10-week carbon tetrachloride (CCl(4) ) challenge, respectively. Injection of polyinosinic-polycytidylic acid (poly I:C) or IFN- induced NK cell activation and NK cell killing of hepatic stellate cells (HSCs) in the 2-week CCl(4) model. Such activation was diminished in the 10-week CCl(4) model. Consistent with these findings, the inhibitory effect of poly I:C and IFN- on liver fibrosis was markedly reduced in the 10-week versus the 2-week CCl(4) model. In vitro coculture experiments demonstrated that 4-day cultured (early activated) HSCs induce NK cell activation via an NKG2D/Rae-1-dependent mechanism. Such activation was reduced when cocultured with 8-day cultured (intermediately activated) HSCs due to the production of TGF- by HSCs. Moreover, early activated HSCs were sensitive, whereas intermediately activated HSCs were resistant to IFN--mediated inhibition of cell proliferation, likely due to elevated expression of suppressor of cytokine signaling 1 (SOCS1). Disruption of the SOCS1 gene restored the IFN- inhibition of cell proliferation in intermediately activated HSCs. Production of retinol metabolites by HSCs contributed to SOCS1 induction and subsequently inhibited IFN- signaling and functioning, whereas production of TGF- by HSCs inhibited NK cell function and cytotoxicity against HSCs. CONCLUSION: The antifibrogenic effects of NK cell/IFN- are suppressed during advanced liver injury, which is likely due to increased production of TGF- and expression of SOCS1 in intermediately activated HSCs. Enhanced liver regeneration in IL-10-deficient mice after partial hepatectomy via stimulating inflammatory response and activating hepatocyte STAT3. Emerging evidence suggests that proinflammatory cytokines, including TNF- and IL-6, play a critical role in the initiation and progression of liver regeneration;however, relatively little is known about the role of anti-inflammatory cytokine IL-10 in liver regeneration after partial hepatectomy (PHx). Here, we examined the role of IL-10 in liver regeneration using a model of PHx in several strains of genetically modified mice. After PHx, expression of IL-10 mRNA in the liver and spleen was significantly elevated. Such elevation was diminished in TLR4 mutant mice. Compared with wild-type mice, IL-10(-/-) mice had higher levels of expression of proinflammatory cytokines and inflammatory markers in the liver, as well as higher serum levels of proinflammatory cytokines after PHx. The number of neutrophils and macrophages was also higher in the livers of IL-10(-/-) mice than in wild-type mice after PHx. Liver regeneration as determined by BrdU incorporation after PHx was higher in IL-10(-/-) mice than in wild-type mice, which was associated with higher levels of activation of IL-6 downstream signal STAT3 in the liver. An additional deletion of STAT3 in hepatocytes significantly reduced liver regeneration in IL-10(-/-) mice after PHx. Collectively, IL-10 plays an important role in negatively regulating liver regeneration via limiting inflammatory response and subsequently tempering hepatic STAT3 activation.